PMR-15 is a commercially-available thermosetting polyimide resin system that has found wide use as the polymer matrix for composite components that require a service temperature of up to about 290.degree. C. (about 550.degree. F.). This particular resin system, whose commercial sources include SP Culver City Composites, has been used to form structural and non-structural components for the aerospace industry, as well as numerous other industries. The general composition of PMR-15 is, in weight percent, about 30 to 40% methylenedianiline (MDA), about 40 to 50% benzophenone tetracarboxylic acid dimethyl ester (BTDE), and about 20 to 30% 5-norbornene-2,3-dicarboxylic acid monomethyl ester (NE). For use, the above mixture is conventionally diluted with a solvent, such as methyl alcohol (methanol), at a weight ratio of about 1:1.
Thermosetting polyimide resin systems of the type exemplified by PMR-15 have a complex reaction system. These resin systems release large amounts of volatiles, such as methanol and water, during the cure process, which complicates the processing and manufacture of quality parts. In addition, utmost care must be taken in the handling of the uncured resin system since it contains incompletely reacted MDA, which is a suspected carcinogen, can cause chemically-induced hepatitis in humans, and is a known kidney and liver toxin. Accordingly, in the processing of PMR-15 resin, precautions must be taken to minimize the risk of exposure to personnel.
The fabrication of composite components from this polyimide resin system typically involves a prepreg, which is a woven reinforcement fabric that has been impregnated with the uncured resin system. Conversion of the resin and a suitable reinforcement structure into a prepreg that meets user specifications is a specialized process performed at a limited number of facilities. Known methods for impregnating the fabric material include solvent dilution, hot melt and powder coating techniques. Prepregs are typically produced to contain about 32 weight percent resin and about 60 volume percent fabric (after cure), with carbon and glass fiber fabrics being most common.
Typical production practices employ sheets of the prepreg on a backing material. Because they are in the uncured state, the prepregs must be kept in cold storage, typically in the form of rolls. The prepreg rolls must therefore be thawed before use, which usually requires about four hours. After thawing, the prepreg can be rolled out onto a cutting table and a portion sufficient to form the desired component is cut from the roll. The roll must then be repacked and returned to cold storage, with the out-time noted to monitor any deterioration of the prepreg. The portions are then cut to form plies which are shaped and oriented with respect to the fabric weave of the prepreg. The plies are numbered, stacked in sequence for production, and placed in a plastic bag to form a kit, which is then ready for transfer to a layup area where the plies are sequentially placed in a mold to form the desired composite component. If the kit will not be used immediately, it must be returned to cold storage. The trimmings, or offal, from the cutting operation must be disposed of in a controlled landfill due to the presence of MDA in the material.
In the layup area, the kit is thawed if necessary, then removed from the bag and placed in ordered sequence on a mold surface. As each ply is positioned, the backing material must be removed and the ply oriented appropriately within the mold cavity according to the part geometry. A debulking operation must typically be performed to remove interlaminar air pockets, typically after every two to four plies are placed on the mold. The layup process is labor intensive, particularly since a number of plies are typically required to form a composite component.
Following the layup process, the component formed by the prepreg plies must be cured on the mold through the application of carefully controlled heat and pressure. The cure process generally involves three stages: imidization, final cure, and post cure. Imidization involves a prescribed heat cycle that is applied through the mold to the component, causing condensation and other reactions to occur by which the constituent monomers of the resin system form an uncross-linked polyimide resin. Upon imidization, the monomer MDA is chemically reacted with other monomers to form the polyimide, and is therefore no longer the previously-noted health hazard. The resulting imidized component may then be removed from the mold, since the use of separate imidization and final cure molds improves the throughput of the fabrication process.
Final cure involves heating the imidized component within the cure mold to a level where the imidized resin remelts and flows, after which pressure is applied to purge entrapped air. While pressure is maintained, the temperature of the imidized component is increased to about 315.degree. C. (about 600.degree. F.) to cause cross-linking of the polyimide, which imparts the desired structural properties for the final component. Post cure involves additional heating of the component at about 315.degree. C. (about 600.degree. F.) to cause additional cross-linking of the polyimide so as to increase its glass transition temperature (T.sub.g), and therefore enhance the thermal properties of the component. This process is typically accomplished by baking the component in a convection oven according to a controlled heating cycle. Final manufacturing processes for the resulting composite component include trimming, machining and inspection, as required.
The complexity and labor intensive nature of processing thermosetting polyimide resin systems can be readily appreciated from the above. Particularly notable disadvantages of this process include the requirement for cold storage of the prepregs, waste and disposal of prepreg trimmings, and exposure hazards to unreacted MDA in the uncured prepreg. Resin transfer molding (RTM) techniques are known by which composite components are formed from reinforcing fibers that are impregnated in-mold with lower temperature resin systems such as epoxies. However, such techniques are not feasible for polyimide resin systems due to the high viscosity of thermosetting polyimide resins and the significant amount of condensation byproducts formed during imidization of the resins, resulting in insufficient fiber impregnation and the formation of voids within the composite component. As opposed to a prepreg which is manufactured by impregnating woven reinforcement cloth with a resin, a tow is a single fiber bundle which is impregnated with resin. Pre-imidized tow has the resin within the tow in an imidized state rather than an uncured state.
Experimentation directed to the use of pre-imidized tow, a single fiber bundle impregnated with imidized resin, has indicated complications in the fabrication of some components, particularly those with axisymmetric structures when using reinforcement fabrics having sheet woven or braided architectures. From such experiments, it has been concluded that the resin bulk (about forty to about fifty volume percent) will inhibit the fabrication of components with through-thickness reinforcement fabric preforms, such as those produced by three-dimensional braiding, knitting and weaving. The degree of compaction required to consolidate a three dimensional processed preform would be substantial and cause significant amounts of distortion or crushing of the reinforcement architecture.
Accordingly, it would be desirable if a less labor-intensive process were available by which polyimide matrix composite components could be formed with a thermosetting polyimide resin system that is imidized with minimal exposure risks to MDA, and the resulting component is capable of retaining its structural integrity at temperatures of up to about 290.degree. C.